[Administrators: feel free to relocate this post at your discretion; with its zoogeographical focus, it could just as aptly lie within the "Evolution" forum.]

Looking to this map of global crayfish distribution, one is immediately struck by what are seemingly several discrepancies - namely, the complete absence of crayfish from a broad pan-tropical belt encompassing most of South America, continental Africa (but not Madagascar) in its entirety, South/Southeast Asia, and [naturally] most of the Indo-Pacific. Based on fossilized remains, it seems likely that crayfish arose in the far-southern reaches of Permian Pangea:

Origins

Discovery of an Early Permian claw from Antarctica extends the fossil record of crayfish by approximately 65 m.y. and demonstrates that decapod crustaceans had radiated into freshwater habitats by the late Paleozoic. Burrows in Lower Triassic rocks of Antarctica are among the oldest apparently constructed by crayfish. Their morphology is similar to modern crayfish burrows, and this demonstrates that burrowing behavior was established early in the evolution of this group. The new discoveries show that the earliest Permian crayfish were distributed in high paleolatitudes of southernmost Pangea, where they lived in freshwater lakes fed by glacial meltwater. Modern crayfish habitat, used as a guide to crayfish temperature tolerance, indicates that summer temperatures of streams and lakes near the South Pole that supported the crayfish probably reached 10-20 degrees C during Permian-Triassic interglacial intervals.

Fossils discovered on an NSF-funded expedition in Antarctica last December show new evidence that freshwater crayfish evolved at least 65 million years earlier than previously thought. Researchers in the Shackleton Glacier area discovered crayfish burrows in 240-million-year-old deposits of the Triassic Period, and identified a fossil claw of the Late Carboniferous-Early Permian Age (285-million-years-old).

The newly found crayfish claw is the oldest known evidence of decapod crustaceans from freshwater deposits anywhere on earth. Crayfish are important components of present freshwater ecosystems in their role as large and abundant omnivores. Their presence in these ancient deposits suggests that freshwater ecosystems resembling those of today developed much earlier than was previously thought. The breakage pattern on the claw, which appears to have been caused by a predator or scavenger, supports this theory in suggesting the presence of a community of species.

As the following quotes reveal, said peculiarity has been considered before, but little of substance has come from this basal speculation:

Several species of prawns (Palæmon) abound in our own seas. Other marine prawns are found on the coasts of North America, in the Mediterranean, in the South Atlantic and Indian Oceans, and in the Pacific as far south as New Zealand. But species of the same genus (Palæmon) [Most freshwater forms are today recognized to belong to either Macrobrachium or Palaemonetes] are met with, living altogether in fresh water, in Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of the Gulf of Mexico, of the West India Islands and of eastern South America, as far as southern Brazil, if not further; in those of Chili and those of Costa Rica in western South America; in the Upper Nile, in West Africa, in Natal, in the Islands of Johanna, Mauritins, and Bourbon, in the Ganges, in the Molucca and Philippine Islands, and probably elsewhere.

Many of these fluviatile prawns differ from the marine species not only in their great size (some attaining a foot or more in length), but still more remarkably in the vast development of the fifth pair of thoracic appendages. These are always larger than the slender fourth pair (which answer to the forceps of the crayfishes) ; and, in the males especially, they are very long and strong, and are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of "Cammarons") are not unfrequently confounded with true crayfishes; though the fact that there are only three pair of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the Astacidæ.

Species of these large-clawed prawns live in the brackish water lagoons of the Gulf of Mexico, but I am not aware that any of them have yet been met with in the sea itself. The Palæmon lacustris (Anchistia migratoria, Heller) abounds in fresh-water ditches and canals between Padua and Venice, and in the Lago di Garda, as well as in the brooks of Dalmatia; but its occurrence in the Adriatic or the Mediterranean, which has been asserted, appears to be doubtful. So the Nile prawn, though very similar to some Mediterranean prawns, does not seem to be identical with any at present known.

In warm climates, however, not only the large prawns which have been mentioned, but Atyæ and fluviatile crabs (Thelphusa) compete for the possession of the freshwaters; and it is not improbable that under some circumstances, they may be more than a match for crayfishes; so that the latter might either be driven out of territory they already occupied, as Astacus leptodactylus is driving out A. nobilis in the Russian rivers; or might be prevented from entering rivers already tenanted by their rivals.

In connection with this speculation, it is worthy of remark that the area occupied by the fluviatile crabs is very nearly the same as that zone of the earth's surface from which crayfish are excluded, or in which they are scanty. That is to say, they are found in the hotter parts of the eastern side of the two Americas, the West Indies, Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan [Indian subcontinent], Burmah [Myanmar], China, Japan, and the Sandwich Islands [Hawai'i]. The large-clawed fluviatile prawns are found in the same regions of America, on both east and west coasts, in Africa, Southern Asia, the Moluccas, and the Philippine Islands; while the Atyidæ not only cover the same area, but reach Japan, extend over Polynesia, to the Sandwich Islands, on the north, and New Zealand, on the south, and are found on both shores of the Mediterranean; a blind form (Troglocaris Schmidtii), in the Adelsberg caves, representing the blind Cambarus of the caves of Kentucky.

As Huxley wrote, prawns of the genus Macrobrachium [with some 200 species, the largest of the Palaemonidae] are almost ubiquitous (suspiciously so with respect to the global range of crayfish) in the warm freshwaters of the world. Representative species range across a broad swath of the globe encompassing Africa (from Mauritania east to Ethiopia and from the lower reaches of the Nile south to the Great Karoo) - that is, the southern- and western- most precincts of the Eurasian biogeographical zone; the Ethiopian zone; and the Central African zone); the Malagasy realm; southern Italy and the neighboring Balkans (Eurasian zone); the Indian subcontinent, Sri Lanka, and Southeast Asia through the Indochinese Peninsula (Oriental zone); southern and western China across the East China Sea to southern Japan (Eurasian zone); the Philippines, Indonesia on both sides of Wallace's Line, New Guinea (and its respective division), and Australia (Austronesian zone); numerous isles of the southern and sub-equatorial northern Pacific Oceans (in rough correlation to those isles of the Polynesian Triangle having permanent freshwater systems [from Samoa to the Marquesas and northeast to Hawaii]; the West Indies (Caribbean transitional zone); Central/South America (Neotopics); southeastern and extreme southwest North America (marginal Palaearctic).

Its seems to me likely that stockily-chelate Macrobrachium prawns and crayfish together constitute a classic case of parallel evolution, on par with the analogous diversification of placental and Australian marsupial mammals.

Look to the following images for some markedly (both outwardly and in general habits) crayfish-like Macrobrachium spp.:

"Krabbenkrebse"/"half cancers" of the genus Aegla (which, despite their appearance, are neither crab nor crayfish but Anomurans affiliated with marine squat lobsters and hermit crabs [see http://www.crusta10.de/index.php?sideid=krabbenkrebse_de&lang=de]) by some accounts forced many South American crayfish into peripheral burrowing roles:

The restricted range of Aegla is very similar to the endemic South American parastacid freshwater crayfish genera, Parastacus, Samastacus, and Virilastacus, and suggests a similar route of colonization. In fact, their ranges overlap so extensively that Riek (1971) suggested that competitive exclusion by aeglids forced crayfish out of streams and rivers and into burrowing lifestyles along river banks and fields.

Experiments centered about conspecific competition between crayfish and Potamonid crabs overwhelmingly indicate the "behavioral dominance" of the latter over the former and suggest a "scenario of competitive exclusion" amongst the two:

1. On the basis of present range and fossil evidence (quite sparse, apparently due to counterintuitively poor preservation of calcaerous chitin), in what timespan of geological time did the genus Macrobrachium arise (alternatively, when did its progenitors colonize freshwater)? In what geographical region (by both historical and present-day continental arrangement) did this transition occur?

2. Is it possible that robust Macrobrachium spp. today comprise an impediment (as per the doctrine of competitive exclusion) to the expansion of crayfish? Are some species of the former grouping "behaviorally dominant" over the latter? Why might this be so? [Macrobrachium spp. certainly possess greater mobility through the water column, typically finer dexterity of the chelipeds, possibly more acutely sensitive chemoreceptive organs, and, in some cases, immediately autonomous benthic larvae.]

3. What past geographical barriers, if any, could have prevented crayfish from having spread to the portions of the continental tropics from which they are absent?

4. Is it plausible that crayfish were once considerably more widespread than at present, and were subsequently displaced by the invasion of freshwater by other decapods?

5. Does anyone know of any formal experiments concerning interspecific competition between crayfish and Macrobrachium to have been conducted?

6. In the regions of Africa where crayfish have been introduced to control disease-harboring aquatic snails or have escaped from aquaculture establishments, have native Macrobrachium (if they exist) proved a hindrance to their advance? Have they, alternatively, lost ground? Is their planktonic larval stage suppressed?

Last edited by Shagreen on Fri Mar 25, 2005 1:06 am, edited 2 times in total.

The Big River Region includes the Missouri and Mississippi rivers. Many kinds of fish are characteristic of the channels of these streams, but crayfish occur only as small local populations or stray individuals [there is presumably no outlier of salinity]. Four-inch shrimp (Macrobrachium ohione) was formerly abundant in the Mississippi River, but disappeared about 30 years ago [The main culprits were unsustainable harvest and artificial channelization of the river; M. ohione has since been "rediscovered" in stretches of its former range]. Several crayfish species characteristic of the other regions are common in sloughs and marshes on the river floodplains [Generally atypical habitat for a fluviate prawn]. These include the White River crayfish (P. acutus) and dwarf crayfish (C. shufeldtii) along the Mississippi River, and the papershell crayfish (O. immunis) along the Missouri River. The devil crayfish (C. diogenes) is the common burrowing crayfish on floodplains of both rivers.

Crayfish-prawn "coexistance" as applies to that regional case, is tenuous at best. However, the text is unclear in one vital regard - while it seems to suggest that crayfish are scarce in streams of the Mississippi/Missouri watersheds of the surveyed region, it gives little mention of the extent of their presence in the river proper (or, conversely, of shrimp colonization of such minor tributaries). If competitive exclusion by riverine prawns was the primary limiting influence, one might expect that the more aquatic species would by today have expanded their range to assume some of the vacated stretches (if dams and pollution would not have impeded such movement).

Note: One must recognize in considering this issue that geographical sympatry is not interchangeable with "side-by-side" coexistance, per se.

A little off the topic but while I went to St louis they didn't have any crayfish to eat. During Mardi Grad there were no crayfish to eat becasue of a storm. Crayfish along with other lake dwllers can be easily wiped out be it a new species added, storms, or ect.